Bought my 55 gallon back in 1986 from a girlfriends brother inlaw. When I first saw the tank it was freshwater and you could bearly see in it with all the puppy chow floating in it. That's right puppy chow, thats what they fed their fish, must have been a 1/2 inch of it settled on the bottom unbelievably nasty.
So:
Saltwater 17 years
Reef 4 years.
Freshwater: ahhhh years and years been nearly 20 years scince I had one. Think I got rid of it after my first salt tank.
Thomas